P-51 has a constant speed propeller. The system tries to maintain a set RPM, which is commanded by the pilot via a lever in the cockpit, by increasing or decreasing the propeller blade pitch angle.
The propeller RPM is limited to 3000, because past that the speed of the blade tip is approaching the speed of sound and the efficiency of the propeller starts to drop rapidly. Because the system is adjusting the angle of the blade, more power allows bigger angle to be set and that pushes more air past the propeller disk, which means more thrust. Even though the RPM of the propeller doesn't change.
All that means that the pilot doesn't need to know what pitch angle he needs to set the propeller to. All he cares are the RPM of the propeller and the engine manifold pressure, which he needs for managing the fuel economy and safe engine operation.